Conversely if the directed radius of curvature in empty space is homogeneous and isotropic Einstein's law will hold. The statement that the radius of curvature is a constant length requires more consideration before its full significance is appreciated. Length is not absolute, and the result can only mean \emph{constant relative to the material standards of length} used in all our measurements and in particular in those measurements which verify $G_{\mu\nu} = \lambda g_{\mu\nu}$. In order to make a direct comparison the material unit must be conveyed to the place and pointed in the direction of the length to be measured. It is true that we often use indirect methods avoiding actual transfer or orientation; but the justification of these indirect methods is that they give the same result as a direct comparison, and their validity depends on the truth of the fundamental laws of nature. We are here discussing the most fundamental of these laws, and to admit the validity of the indirect methods of comparison at this stage would land us in a vicious circle. Accordingly the precise statement of our result is that the radius of curvature at any point and in any direction is in constant proportion to the length of a specified material unit placed at the same point and orientated in the same direction. This becomes more illuminating if we invert the comparison--- \emph{The length of a specified material structure bears a constant ratio to the radius of curvature of the world at the place and in the direction in which it lies}.\hfill\tTag{(66.3)}\quad\null The law no longer appears to have any reference to the constitution of an empty continuum. It is a law of material structure showing what dimensions a specified collection of molecules must take up in order to adjust itself to equilibrium with surrounding conditions of the world.